/Engine/Source/Runtime/Slate/Public/Framework/Docking/TabManager.h
Set the multi-box to use for generating a global menu bar. The implementation is platform and setting specific On Mac the menu bar appears globally at the top of the desktop in all cases regardless of whether or not SetAllowWindowMenuBar is called. On other desktop platforms the menu appears at the top of the window this tab manager is a part of only if SetAllowWindowMenuBar(true) is called.
void SetMenuMultiBox ( const [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [FMultiBox](API\Runtime\Slate\Framework\MultiBox\FMultiBox) > NewMenuMutliBox, const [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > MenuWidget )